The present invention relates generally to control systems and more specifically to an ignition control system for a gas appliance, such as a furnace having a direct spark ignition system.
In the typical direct spark ignition system, a main gas valve is opened and an ignition assembly ignites the gas flowing out of the main gas valve into a combustion chamber. The gas is ignited directly by an electronic source of heat, such as a spark generator or a high-temperature resistor, rather than a pilot flame, which provides heat for predetermined time intervals.
A flame probe typically detects whether the ignition assembly has succeeded in igniting the gas. If the ignition assembly has not been successful, or if a flame already established goes out, the main gas valve is closed. In many systems, the valve may not then be reopened until a predetermined period of time has elapsed. This period of time allows a blower to purge any residual gas in the combustion chamber before the valve is reopened and a spark occurs within the chamber. Otherwise, a large amount of gas may have collected within the chamber, and introducing a spark in the chamber before the gas is removed may result in a harmful explosion.
Clearly, the timing of the valve, ignition assembly, and blower is an important feature of safely operating a gas appliance such as a direct ignition furnace. The control for such timing must be very reliable. Generally, the gas valve must be closed when the system malfunctions because of an unknown cause. Otherwise, the gas may continue to flow into the combustion chamber and surrounding areas, even though no flame exists. Thus, gas could fill the chamber (or entire dwelling associated with the appliance) and, upon reaching a critical level, explode.
Thus, if a malfunction occurs within the appliance or within the control itself, the control should revert to failsafe status where the valve is shut. Moreover, if a serious fault occurs, the control should keep the valve closed, or in a "lockout" condition.
Nonetheless, the control must not unnecessarily shut off the valve or go to a "lockout condition." Such unnecessary closing of a valve is, of course, inconvenient to those using the appliance. The control should especially avoid an unnecessary valve closing where the valve is used in a furnace in a cold climate. An unnecessary valve closure could prevent the furnace from heating the associated building. In cold weather, such a shutdown of the furnace could result in broken pipes in the associated building (when the water within the pipes freezes and expands) or even loss of life.
Aside from closing the gas valve when required to lessen the risk of explosion, but not more often than necessary, the control for the appliance should be as inexpensive and easy to use as possible. In this way, the cost of gas appliances may be reduced for consumers, and users of the control will be less likely to operate it incorrectly.
Moreover, the control should have as few discrete components as possible. In this way, the risk that the control has been assembled incorrectly or will malfunction during operation may be reduced.